Investigating the use of ecozones in the design of water balance covers for waste containment by analyzing the sensitivity of cover effectiveness to climate, vegetation and soil parameters

Abstract

Water-balance landfill covers are a relatively new and innovative design for waste containment that does not require the previously mandated clay layers or geomembranes that are expensive to manufacture or import. The designs of these covers are often based on one-dimensional unsaturated flow modeling to simulate water flux into and out of the cover. Water balance covers are engineered to minimize percolation through the bottom of the cover by managing the storage and removal of water in the cover. The covers store water from precipitation events during the wet seasons and that water is subsequently released due to evapotranspiration in the warmer growing seasons. It has been widely accepted that detailed site specific investigations are required to fully characterize the climate, vegetation and soil parameters that determine the effectiveness of water balance covers. Recent research (Cadmus, 2011) has suggested that the design of water balance covers can be done in a more generic manner, using "ecozones," or regions of similar climate and geographic properties, to define the parameters used in modeling. It is important to understand to what extent it is viable to use ecozones in lieu of site-specific characterization. For this project a sensitivity analysis is conducted on select climate, vegetation, soil parameters and the timing of precipitation patterns using a combination of HYDRUS 1-D and UCODE. It was found that cover effectiveness is highly sensitive to soil parameters, particularly saturated hydraulic conductivity and the fitting parameters in the van Genuchten equation, [alpha] and n. Water balance covers were also found to be sensitive to the timing of precipitation events in relation to the growing season and periods of maximum PET. Cover effectiveness has shown to be highly sensitive to a variety of parameters that vary significantly within large ecozones. A monte carlo analysis was conducted for two ecozones in Colorado, USA (the front range and western slope). At each ecozone it was determined that variability in climate, vegetation and soil parameters is significant enough to effectively rule out generic cover methodology for the two ecozones. Due to the large variability and uncertainty in climate, vegetation and soil parameters within an ecozone, the methodology of designing water balance covers in a generic manner based on ecozones is demonstrably questionable.